1. Field of the Invention
This invention relates to an optical transmission system for analog or digital radio frequency (RF) signals using an externally modulated solid-state laser, and in particular to an optical modulator fabricated with and coupled to such laser.
2. Description of the Related Art
An optical telecommunication system transmits information from one place to another by way of an optical carrier whose frequency typically is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, light beam, or a lightwave signal. The optical telecommunication system includes several optical fibers and each optical fiber includes multiple channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. The purpose for using multiple channels in the same optical fiber (called dense wavelength division multiplexing (DWDM)) is to take advantage of the high capacity (i.e., bandwidth) offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap. International Telecommunications Union (ITU) standards currently determines the channel separations.
One link of an optical telecommunication system typically has a transmitter, the optical fiber, and a receiver. The optical transmitter has a laser, which converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
Optical transmitters for the transmission of analog or digital radio-frequency (RF) signals over an optical fiber may use either a directly modulated laser or a continuous wave (CW) laser coupled to an external modulator.
Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as voice and video signals, over optical fibers. Although such analog transmission techniques have the advantage of substantially smaller bandwidth requirements than digital transmission, such as digital pulse code modulation, or analog or pulse frequency modulation, the use of amplitude modulation typically places more stringent requirements on the noise and distortion characteristics of the transmitter. A limiting factor in such links can be the second order distortion due to the combination of optical frequency modulation, or chirp, and fiber dispersion.
For these reasons, direct modulation techniques have typically been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with low dispersion. It is also possible to use direct modulation of 1550 nm lasers, but in this case the distortion produced by chirp and dispersion must be cancelled using a predistorter that is set for the specific fiber length. In some case, such as when the signal must be sent to more than one location or through redundant fiber links of different length, such a programmable predistorter can be undesirable.
Stimulated Brillouin scattering (SBS) effects that depend on the optical launch power and the total fiber length may also degrade DWDM system performance. SBS is an opto-acoustic nonlinear process that can occur in single mode optical fibers. This optically induced acoustic resonance effectively limits the amount of optical power that can be successfully transmitted through the single mode optical fiber.
The SBS can perhaps be best explained in terms of three waves in an optical fiber. When an incident wave (also known as “pump wave”) propagating along the optical fiber reaches a threshold power (which may vary), it excites an acoustic wave in the optical fiber. The optical properties of the optical fiber such as the refractive index are altered by the acoustic wave, and the fluctuation in the refractive index scatters the incident wave, thereby generating a reflected wave (also known as “Stokes wave”) that propagates in the opposite direction.
Because of the scattering, power is transferred from the incident wave to the reflected wave, and molecular vibrations in the optical fiber absorb the lost energy, because of which, the reflected wave has a lower frequency than the incident wave. Hence, the scattering effect can result in attenuation, power saturation and/or backward-propagation, each of which deteriorates the DWDM system performance. Hence, the attenuation is caused by the transfer of power from the incident wave to the acoustic and reflected waves. Due to power saturation, there is a limit to the maximum amount of power that can be transmitted over the optical fiber. Also, the backward propagation wave can create noise in transmitters and saturate amplifiers.
The phenomenon of SBS has been known by optical network equipment designers for a number of years. Essentially, SBS results when a threshold power level is exceeded within a sufficiently narrow frequency band in a fiber optic light guide. The increasing operational relevance of SBS relates to the development of lasers such as, for example, single longitudinal mode lasers which readily provide an output that exceeds the SBS threshold (typically about 4 mW in, for example, a 50 kilometer fiber optic cable). Moreover, limitation of optical power to a level as low as 4 mW not only fails to utilize the output power available from state of the art lasers, but limits distance transmission through fiber optic cable by an unacceptable margin.
Various approaches to minimize the effect of SBS are also known. In general, SBS impact can be reduced in an externally modulated analog system if the optical signal's spectrum can be broadened since the energy per bandwidth is lowered. Some effective and widely used techniques for combating SBS include the use of an optical phase modulator or dithering the laser or the combination of both, in the case of external modulators.
To avoid the distortion problems related to chirp and dispersion at 1550 nm with direct modulation, low chirp external optical modulators are commonly used in analog fiber optic communication systems, such as CATV signal distribution, to amplitude modulate an optical carrier with an information or content-containing signal, such as audio, video, or data signals.
Since the present disclosure also relates to external optical modulators associated with a laser, a brief background on external optical modulators is noted here. There are two general types of external optical modulators implemented as semiconductor devices known in the prior art: Mach Zehnder modulators and electro-absorption modulators. A Mach-Zehnder modulator splits the optical beam into two arms or paths on the semiconductor device, one arm of which incorporates a phase modulator. The beams are then recombined which results in interference of the two wavefronts, thereby amplitude modulating the resulting light beam as a function of the modulated bias signal applied to the phase modulated arm. An electro-absorption modulator is implemented as a waveguide in a semiconductor device in which the absorption spectrum in the waveguide is modulated by an applied electric bias field, which changes the band gap energy in that region of the semiconductor, thereby modulating the amplitude or intensity of the light beam traversing the waveguide.